The present invention pertains to thermally shock resistant ceramics, and more particularly to a ceramic honeycomb structures such as ceramic catalyst supports or particulate filters having high thermal shock resistance, useful for example in thermally demanding applications such as motor vehicle exhaust gas catalyst supports and soot filters.
Wall-flow diesel particulate filters (DPFs) and ceramic catalyst supports generally comprise thin-walled ceramic honeycomb structures with high geometric surfaces areas and, in some cases, with extensive interconnected porosity for good fluid filtration. Ceramic filters, in particular, must exhibit high mechanical strength for handling and superior thermal shock resistance in operation.
The thermal shock resistance (TSR) of a composite ceramic body is related to the stress at fracture (as given by strength at fracture MOR or Modulus of Rupture), the elastic modulus (E) and the strain at fracture i.e. the product of thermal expansion coefficient (α) and thermal gradient (ΔT), through the following expression:
      T    ⁢                  ⁢    S    ⁢                  ⁢    R    ∝            M      ⁢                          ⁢      O      ⁢                          ⁢      R                      E        ·        α        ·        Δ            ⁢                          ⁢      T      
High levels of thermal shock resistance have typically been sought by attempting to obtain extremely low values of thermal expansion coefficient, or through methods to reduce thermal gradients during high temperature use. However, it can be seen from the expression above that use of a material with a sufficiently high strength and a sufficiently low bulk elastic modulus (i.e. an increased strain tolerance) could offer adequate thermal shock resistance for some applications.
Cordierite is known for its low thermal expansion coefficient, high refractoriness, porosity control, and ease of processing in applications for emissions control devices such as catalyst supports and diesel particulate filters. Advantageously, most cordierite honeycomb bodies being manufactured today achieve a lower bulk thermal expansion coefficient than would be expected from the crystallographic properties of cordierite, due to crystal orientation in the bodies and the presence of microcracks in the cordierite material. Microcracks can arise from stresses resulting from the anisotropic thermal expansion of regions of oriented cordierite crystallites (domains) during the cooling step of the firing process. During subsequent heating, the thermal expansion of the cordierite is lowered when the microcrack voids act as expansion joints, giving a bulk thermal expansion coefficient lower than that of a body with no microcracks. On cooling, the cracks reopen in a reversible process.
Microcracks can be beneficial to thermal shock resistance in many applications, but also are accompanied by certain drawbacks. One drawback is that the strength of the body is reduced by the presence of microcracks. Another is that the bulk coefficient of thermal expansion is sensitive to the processes that alter the way the microcracks heal and reopen during use.
One way the microcracking effect on thermal expansion coefficient can be interrupted is by the intrusion of foreign matter into the crack. It has been found that catalyst washcoat materials, particulates such as soot present in diesel engine exhaust gases, and residual ash material arising from inorganic constituents present in fuels and thus the exhaust streams of combustion engines can be introduced into open microcracks of ceramic filters or catalyst supports. This foreign matter can act as pillars and/or wedges in the microcracks of the ceramic, reducing or eliminating those cracks as factors in lowering the bulk thermal expansion and the elastic modulus of the structures. The net result is a reduction in thermal shock resistance.
Because of these pillaring effects, it would be considered an advancement in the art of DPF and catalyst support applications if a composite ceramic body could be developed having a high degree of thermal shock resistance in the absence of microcracks.